1. Field of the Invention
The present invention generally relates to electron beam exposure devices, and more particularly, to an electron beam exposure device including a thermoelectron emission type electron gun which is capable of emitting an electron beam upon heating.
2. Description of the Related Art
Recently, demands such as precise forming of an LSI (large-scale integration) circuit pattern which may reliably correspond to a fine structure of an LSI circuit and a high-speed forming of the fine LSI circuit pattern in order to improve a throughput of the circuit have increased for an electron beam exposure device.
In the IC processing technology, it is known that photolithography may form a minimum pattern width of about 0.3 .mu.m whereas the electron beam irradiation technique may achieve a pattern width less than 0.1 .mu.m. Therefore, it is more desirable and suitable to use an electron beam exposure device to form highly integrated circuit patterns.
An electron gun is a device, comprising a series of electrodes, that produces an electron beam, usually a narrow beam of high-velocity electrons whose intensity may be controlled by electrodes in the gun.
There are mainly two types of electron gun, one is a thermoelectron emission type and the other is a field emission type. In the thermoelectron emission type electron gun, thermoelectrons are released from a thermoelectron generating source, which is made of an electron emitting material such as LaB.sub.6 crystal, when it is heated to about 1500.degree. C. and an electron beam is formed by accelerating the emitted thermoelectrons. In the field emission type electron gun, a needle-like electron generating source whose curvature at a tip is about a few angstrom is provided, and a high electric field is formed around the tip of the electron generating source by applying about -10 to about -50 volts. The high electric field thus formed is capable of reducing the potential barrier at the surface of the emitter and allows electrons to escape from the surface. The distortion of the potential barrier at sufficiently large values of the accelerating field results in an effective narrowing of the barrier and allows the tunnel effect to operate, so liberating more electrons. The electrons thus released are accelerated and an electron beam is formed.
In the electron beam irradiation, a time required for the irradiation is inversely proportional to a current density used. Therefore, it is desirable to employ an electron beam of high current density in order to obtain high throughput of an IC production. Since an electron beam of higher current density is obtainable by the thermoelectron emission type electron gun, compared with the field emission type electron gun, it is preferable to use the thermoelectron emission type electron gun in the production of integrated circuits.
Also, it is important in the practical process of the IC production to improve the operation efficiency of the electron beam exposure device. For instance, the electron beam irradiation operation should be restarted as soon as possible after the completion of maintenance processes. As will be described later, it is necessary to take into account the effect of heat which is used to heat up the thermoelectron generating source to about 1500.degree. C. when restarting the electron beam irradiation.
FIG. 1 is a schematic diagram for explaining a conventional electron beam exposure device 10. The electron beam exposure device 10 is comprised of a mirror cylinder 11, a thermoelectron emission type electron gun 12, a vacuum chamber 13, a stage 14 and a control circuit 15. The thermoelectron emission type gun 12 is provided at the upper portion 48 of the mirror cylinder 11 and the stage 14 is located in the vacuum chamber 13 which is provided at the bottom of the mirror cylinder 11. In the mirror cylinder 11, an alignment coil 20, a first electromagnetic lens 21, an aperture 22, a second electromagnetic lens 23, a third electromagnetic lens 24, an aperture 25, a fourth electromagnetic lens 26 and a deflector 27 are provided in that order from the top. A Faraday cylinder (Faraday cup) 28 for measuring current of an electron beam is provided at a predetermined position of the stage 14.
The control circuit 15 is comprised of a computer unit 30, an electron gun driving circuit 31, a pattern correction circuit 33 and a stage control unit 34.
FIG. 2 is a diagram showing a cross section of the electron gun 12 in FIG. 1 in a magnified scale. As shown in FIG. 2, the electron gun 12 is comprised of a cathode 40, an anode 41, a cathode heating element 42, a supporting member 43, a Wehnelt 44, a base 45, a cover 46 and an insulating oil 47. The cathode 40 may be made of a LaB.sub.6 crystal and the cathode heating element 42 is provided on both sides of the cathode 40. The supporting member 43 supports the cathode 40 and the cathode heating element 42 and the Wehnelt 44 is provided so as to surround and support the support member 43. The Wehnelt 44 is fixed to the base 45 having a substantially inverted cone shape, which may be made of ceramics, and the cover 46 of a plate shape is provided at the top of the base 45 in which the insulating oil 47 is contained.
The portion of the thermoelectron emission type electron gun 12 around the cover 46 is fixed to the upper portion 48 of the mirror cylinder 11 so that the thermoelectron emission type electron gun 12 is entirely contained in the mirror cylinder 11.
In the electron beam exposure device 10 comprising the thermoelectron emission type electron gun 12 which has the configuration mentioned above, thermoelectrons are released from the cathode 40 when the electron gun driving circuit 31 supplies a current, i.sub.1, for heating the cathode 40 to the cathode heating element 42 so as to heat up the cathode heating element 42 through which about five to 12 watts of heat quantity are given to the cathode 40 so that the temperature of the cathode 40 may be increased to about 1520.degree. C. The released thermoelectrons are accelerated by the anode 41 so that an electron beam 49 is formed, and the electron beam 49 may be irradiated onto a wafer 50 having been coated with a resist and fixed on the stage 14, so that a patterning operation may be performed.
As indicated by the arrows 55 in FIG. 2, the heat generated by the cathode heating element 42 is also transferred to the supporting member 43, the Wehnelt 44 and the base 45 and increases the temperature thereof. Also, the heat transferred to the supporting member 43 and the Wehnelt 44 passes through the base 45 and is transferred to the upper portion 48 of the mirror cylinder 11 where it is emitted to the surrounding atmosphere.
However, when the temperature of the Wehnelt 44 of the thermoelectron emission type electron gun 12 is changed, factors such as the positional relationship between the cathode 40 and the Wehnelt 44, and the setting angle of the electron gun 12 is gradually varied due to the thermal expansion of the Wehnelt 44. Thus, the emission angle or the emission direction of the electron beam is changed and the crossover position is varied.
Also, when the temperature of the base 45 and the upper portion 48 of the mirror cylinder 11 is changed, the incident position and the incident angle of the electron beam emitted from the electron gun 12 with respect to the alignment coil 20 is varied. Thus, the deflection property of the alignment coil 20 is changed, and ultimately the irradiation position of the electron beam to the wafer 50 and the current density of the electron beam are varied.
If any of the crossover position, the irradiation position and the current density of the electron beam are changed by the cause as mentioned above in the thermoelectron emission type electron gun 12, the accuracy of the electron beam irradiation with respect to the wafer 50 is lowered.
Therefore, it is necessary to carry out the maintenance operation of the electron beam exposure device 10 as mentioned above. Although it is preferable to restart the electron beam exposure device 10 immediately after the maintenance operation in order to improve the efficiency in the IC production, it is necessary to wait for a temperature stabilization of the Wehnelt 44, the base 45 and the upper portion 48 of the mirror cylinder 11 for the reasons mentioned above. That is, it is necessary to wait for the electron gun 12 to reach the thermal equilibrium state in order to restart the electron beam irradiation.
FIGS. 3A through 3F are diagrams for explaining the steps through which the temperature of the entire electron gun 12 reaches a thermal equilibrium state when heated by the heating element. In FIGS. 3A through 3F, the time, t.sub.1, at which the temperature of the cathode 40 of the electron gun 12 reaches 1520.degree. C. heated by the cathode heating element 42, is used as a criterion. It is shown in the figures that as the time is elapsed (t.sub.2, t.sub.3, . . . ), the temperature of the Wehnelt 44 and the base 45, respectively, is gradually increased and the temperature distribution of the electron gun 12 is changed in order of I.fwdarw.II.fwdarw.III.fwdarw.IV.fwdarw.V and reaches the thermal equilibrium state indicated by a line VI at time t.sub.6 as shown in FIG. 3F. After the time t.sub.6, the thermal equilibrium state indicated by the line VI of the electron gun 12 may be maintained in the electron beam exposure device 10.
In the above steps shown in FIG. 3A to 3F, the total time from time t.sub.1 to time t.sub.6 is about 12 hours, which is considered to be significantly large, due to the large heat capacity of the electron gun 12 including the upper portion 48 of the mirror cylinder 11. In the practical process, a final checking and adjustment operation of an optical system is carried out after the time t.sub.6, and the electron beam irradiation is started.
Thus, in the actual operation of the electron beam exposure device 10, more than one day to is required to restart the device 10 after the completion of the maintenance operation. Accordingly, the operation efficiency of the electron beam exposure device 10 is low and the throughput of the IC production is decreased.
FIG. 4 is a graph showing the temperature change of the electron beam exposure device 10 after its operation is started (i.e., the time t.sub.0 at which the power source is switched on and the cathode heating element 42 is about to generate the heat--this is slightly before the above-mentioned time t.sub.1.) The temperature of the outside of the electron beam exposure device 10 is measured. The line indicated by X(P.sub.1) indicates the temperature change of the position P.sub.1 of the upper portion 48 of the mirror cylinder 11 as shown in FIG. 1 and the line XI(P.sub.2) indicates the temperature change of the position P.sub.2 on the mirror cylinder 11. Likewise, the line XII(P.sub.3) indicates the temperature change at the position P.sub.3 in a room where the electron beam exposure device 10 is located. As can be seen from the graph, the temperature of the upper portion 48 of the mirror cylinder 11 indicated by the line XI(P.sub.1) is stabilized after 12 hours from the time t.sub.0.